This invention relates generally to an electrostatographic printing machine, and more particularly concerns an improved development system for use therein.
An electrostatographic process involves the formation and utilization of electrostatic latent charge patterns for the purpose of recording and reproducing the patterns in viewable form. The field of electrostatography includes electrophotography and electrography. Electrophotography is that class of electrostatography which employs a photosensitive medium to form, with the aid of electromagnetic radiation, the electrostatic latent charge pattern. Xerography, which employs infrared, visible or ultraviolet radiation, and xeroradiography, which employs x-rays or gamma rays, are sub-classes of electrophotography. Electrography is that class of electrostatography which utilizes an insulating medium to form, without the aid of electromagnetic radiation, the electrostatic latent charge pattern. Xero printing, which uses a pattern of insulating material on a conductive medium to form electrostatic charge patterns, and electrographic recording, which uses a charge transfer between a plurality of electrodes to form directly, electrostatic charge patterns, are sub-classes of electrography. Development, which is the act of rendering an electrostatic pattern or image viewable, is employed in all of the aforementioned classes of electrostatography. In the illustrated embodiment hereinafter discussed, an electrophotographic process is utilized.
An electrophotographic process involves the use of a photosensitive element having a photoconductive insulating layer which is charged to a substantially uniform potential in order to sensitize its surface. The charged photoconductive surface is exposed to a light image of an original document to be reproduced. As a consequence of the exposure, the charge is selectively dissipated in the irradiated areas in accordance with the light intensity reaching the surface, thereby creating an electrostatic latent image on the photoconductive surface. Development of the electrostatic latent image recorded on the photoconductive surface is achieved by bringing the charged photoconductive surface into contact with a developer mix. Typical developer mixes employed are well-known in the art, and generally comprise dyed or colored thermoplastic powders, known in the art as toner particles, which are mixed with coarser carrier granules, such as ferromagnetic granules. The toner particles and carrier granules are selected such that the toner particles acquire the appropriate charge relative to the electrostatic latent image recorded on the photoconductive surface. When the developer mix is brought into contact with the charged photoconductive surface, the greater attractive force of the latent electrostatic image recorded thereon causes the toner particles to transfer from the carrier granules and adhere to the latent electrostatic image. This concept was originally disclosed by Carlson in U.S. Pat. No. 2,297,691 and is further amplified and described by many related patents in the art.
Many factors influence the quality of the developed image, the most significant factor being the uniformity with which the toner particles are deposited on the electrostatic latent image recorded on the photoconductive surface. Heretofore, development systems have employed rotary impellers, fur brushes, bucket conveyors and magnetic brush systems to achieve the requisite uniformity in toner deposition. The magnetic brush system achieves a high degree of uniform toner deposition, and, therefore, numerous electrostatographic printing machines utilize this type of development system. Magnetic brush systems usually include a developer roll having a directional flux field adapted to bring the magnetizable developer mix into contact with the charged photoconductive surface. Multicolor electrostatographic printing involves the utilization of various processing components adapted to produce a series of electrostatic latent images corresponding to a particular color of the original. In such a system, there is a need to develop successive partial color images. Each partial color image is developed with toner particles corresponding in color to the partial color image utilized to form the respective electrostatic latent image on the photoconductive surface.
Generally, the developer roll of the magnetic brush development system is mounted fixedly relative to the photoconductive surface. This places a practical limitation on the quality of multi-color copies. A multi-color development system utilizes a plurality of developer rolls, each being adapted to furnish the appropriate colored toner particles to the photoconductive surface. Hereinbefore, fixedly mounted developer rolls were closely spaced to the photoconductive surface, thereby permitting the operative developer roll, i.e. the developer roll having the developer mix adhering thereto. However, when a developer mix having toner particles of one color contacts a toner powder image of another color, intermingling of colors and mechanical scraping occurs. This results in the toner powder image being mis-colored, and the multi-color copy produced thereby lacking the appropriate color balance, i.e. the color does not correspond to the original to be copied. Therefore, it is apparent that the aforementioned type of magnetic brush development system, wherein each developer roll is fixedly mounted, does not produce high quality multi-color copies.
Accordingly, it is a primary object of the present invention to improve the development system of a multi-color electrostatographic printing machine.